1. Field of the Invention
The present invention relates generally to hand held instruments and more particularly to a writing, marking or similar implement in which a replaceable unit is reciprocated between a concealed position and a point exposed position in response to relative rotation of two sections of the implement housing.
2. Description of the Prior Art
The prior art is replete with proposals for instruments of the type having a manually operated mechanism for reciprocating a writing or marking unit between a concealed position and a point exposed position. These proposals can be catagorized generally into two basic groups. One group includes a push button or similar exposed element adapted to be pressed by the user to actuate the mechanism and move the unit. In the other group, the instrument holder includes a pair of coaxially aligned sections which are rotated relative to one another to actuate the mechanism and reciprocate the unit.
One of the features or advantages of the usual push button type instrument is the ease and convenience with which the cap and barrel are disassembled for removal or replacement of a spent or damaged unit. Almost without exception, in this type of instrument, the cap and barrel are coupled by a simple threaded connection adapted to be engaged and disengaged by a universally recognized and easily understood turning action.
In the past, attempts have been made to use the same simple type of threaded connection to releasably couple the cap and barrel of an instrument having a rotating type mechanism. Unfortunately, these attempts have met with little or no success. Repeated actuations of a rotating type mechanism will tend to "unlock" the usual threaded connection even if it initially is tight, thereby resulting in inadvertent loosening of the cap and barrel. Thus, prior to the present invention, the advantages of a simple threaded cap/barrel connection have not been available in instruments having a rotating type mechanism.
One proposal for overcoming this problem of inadvertent loosening is described in U.S. Pat. No. 3,292,592. Essentially the structure suggested in this patent includes a coarse thread for the actuating mechanism and a fine thread for the cap/barrel coupling. The coarse thread is described as having less "lock up" force than the fine thread, whereby the mechanism supposedly will operate with less rotary force than that required to loosen the cap/barrel coupling. However, this patent does not recognize that even if the fine coupling threads are properly "locked up", repeated actuations of the mechanism will tend to loosen them, just as repeated taps on a stubborn jar lid will gradually loosen the lid.
As the writing instrument industry has not previously come up with a practical solution to this "loosening" problem, most commercially available rotary mechanism type instruments have utilized various "rear loading" structures, several of which are illustrated in the following representative U.S. Pats. Nos.: 2,753,844; 3,436,160; 3,027,873; 3,630,629; 3,289,637.
In this general type of structure, removal of the cap exposes an adapter carried on the rearward end of the writing unit for releasable connection to a reciprocal follower portion of the mechanism. If it is desired to replace the unit, the adapter is unscrewed or detached from the follower and the unit withdrawn therethrough. The new unit is then inserted through the mechanism, and the adapter of the new unit attached to the follower. Reassembly of the cap then completes the unit replacement process.
Such "rear loading" instruments have had some commercial success, but not without creating additional problems. For instance, it has been necessary to limit their use to the so-called "capillary" type small diameter units which can be assembled through the mechanism. Otherwise, use of a large jumbo type unit would require enlargement of the diameter of the mechanism and holder to the point where the finished instrument would be uncomfortable and unwieldy for most users. Also, since the unit is attached to the mechanism follower, a special threaded adapter must be provided with each unit. This adapter not only increases the ultimate cost of the unit, but prevents its use in other types of instruments. Further, the process of replacing the unit is time consuming and complicated. Yet another problem is the necessity of having a unit of the proper size and with the proper adapter. If such a unit should not be available, other types or brands generally cannot be used. In addition, the usual "rear loading" type rotating mechanism is complicated and costly to produce and repair.